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The heat transfer characteristics of supercritical China RP-3 aviation kerosene flowing downward in a vertical circular tube are numerically investigated. A ten-species surrogate model is used to calculate the thermophysical properties of kerosene and the Re-Normalization Group (RNG) k-ε model with the enhanced wall treatment is adopted to simulate the turbulent flow. The effects of diameter, wall heat flux, and pressure on temperature and heat transfer coefficient are studied. The numerical results show three types of heat transfer deterioration exist along the flow direction. The first deterioration at the tube inlet region is caused by the development of the thermal boundary layer, which exist whatever the operation condition is. The second and third kind of deterioration take place when the inner wall temperature or the bulk fuel temperature approaches the pseudo-critical temperature under a pressure close to the critical value. The heat transfer coefficients increase with decreasing diameter and increasing pressure. The increase of inlet pressure can effectively eliminate the deteriorations because the thermophysical properties change less near the critical point at higher pressure. The decrease of wall heat flux will delay the onsets of the second and third kind of deterioration. The numerical heat transfer coefficient fit well with the empirical correlations.

Heat transfer and pressure drop characteristics of mini smooth and microfin tubes were studied numerically using water as working fluid at Reynolds number ranging from 7500 to 17500. Seven microfin tubes were used with the same inner diameters of 4.6 mm and 18° helix angle and with number of fins ranging from 30 to 50, fin apex angle ranging from 10° to 40°, and fin height ranging from 0.1 to 0.15 mm. The numerical results fit well with the empirical correlations for heat transfer coefficients and pressure drops. The results indicate that the j-factor of the microfin tubes is approximately 1.2∼1.4 times of that in smooth tubes at the same Re. The j-factor increases with increasing number of microfin and the microfin height and with decreasing fin apex angle. The f-factor of the microfin tubes is approximately 1.05∼1.25 times of that in the smooth tube at the same Re, and the difference between the factors increases with the Re rising. The performance evaluation criterions (PEC) of the seven microfin tubes ranges from 1.15 to 1.35, indicating that microfin tubes exhibit better comprehensive performance compared with smooth tubes. The fluid at the center has a strong tendency to move towards the heated wall along the radial direction due to the directing effect of the microfins. The distinctive flow pattern in the radial direction can sufficiently enhance the turbulent flow near the wall and strengthen the mixing between the cold fluid at the center and hot water at the wall, leading to the enhancement of heat transfer in the near-wall region.

Heat transfer characteristics of aviation kerosene were investigated numerically using a three-dimensional model. The influence of the ratio of longer axis to short axis of elliptical tubes (r), inlet Reynolds number (Re), and pressure (P) of kerosene on local heat transfer characteristics were discussed in detail. The results indicate that the heat transfer coefficient (h), wall temperature (Tw), and bulk fluid temperature (Tb) increase along Z axis direction from inlet to outlet when P=4Mpa. h increases with increasing r and Re, while Tw decreases with increasing Re and r. Tb decreases with increasing Re and remains stable for a varying r. For P=3Mpa, the deterioration of heat transfer occurs near the outlet of computational geometry, where the temperature of near wall region fluid exceeds the critical point and specific heat decrease dramatically. A correlation for heat transfer coefficient of aviation kerosene at supercritical pressure inside elliptical tubes is proposed and compared with the present simulation data. It is shown that this correlation can predict the simulation data within an ±15% error band. Compared with circular tubes, elliptical tubes can enhance the heat transfer effect and decrease the wall temperature, thus enhance the security of the operating system.